1. Field of the Invention
The present invention relates generally to a golf club shaft and a method of producing the same. More particularly, the present invention relates to a golf club shaft of which kick point position can be adjusted as desired and a method of producing a golf club shaft of the foregoing type of which kick point position can easily be adjusted without any deterioration of properties of the golf club shaft.
2. Statement of the Related Art
A golf club shaft has been variously improved from the viewpoints that the ball flying distance is elongated, the locus of ball flying is changed, and the directionality of flying of the ball is stabilized.
A variety of researching activities have been conducted with respect to a kick point of the golf club shaft i.e., a position where the golf club shaft easily flexes. For example, when the kick point of the golf club shaft is located on the head side (tip side), the ball is easy to fly highly, and the high locus of flying of the ball is easily described. On the other hand, when it is located on the grip side (butt side), the directionality of flying of the ball is stabilized. Since the aforementioned facts are clarified, the kick point of the golf club shaft has been changed in a various manner. Various methods are thinkable as a method of adjusting the position of the kick point. One of the methods is a filament winding method, i.e., a method of producing a golf club shaft wherein filaments each impregnated with a thermosetting resin are wound around a mandrel at a predetermined angle, and thereafter, the thermosetting resin is cured. With respect to the foregoing method, there is known a method of adjusting the kick point by changing the angle for winding the filaments at the kick point position so as to allow them to be easily bent (an angle of .theta. shown in FIG. 3 to be described later is set to 20.degree. on the butt side as well as on the tip side and it is set to about 40.degree. at the position in the vicinity of the kick point). In this case, there arises a drawback that a bending strength of each filament becomes weak in the region where the foregoing angle has been changed.
A golf club shaft having its kick point changed by forming a fiber layer by filament winding, and thereafter, forming a reinforcement layer by partial sheet winding is disclosed (refer to Japanese Utility Model Laid-Open Publication No. 63-133261).
Such golf club shaft is produced by forming a fiber layer 2 by winding filaments around a mandrel 1, and thereafter, Partially winding a reinforcement layer 3 on the fiber layer 2 by employing a sheet winding process, moreover, forming a fiber layer (not shown) along the whole length of the reinforcement layer 3, and subsequently, allowing the plural layers to be cured and then disconnecting the mandrel 1.
With the golf club shaft produced in that way, since the reinforcement layer 3 is formed by employing the sheet winding process, there arises a drawback that a joint portion for the reinforcement layer is formed about the circumferential part and the golf club shaft exhibits directionality attributable to the presence of the joint portion. In addition, since filaments are wound around the reinforcement layer 3 again after a sheet is wound around the reinforcement layer 2, there arises other drawback that a filament winding machine should be installed together with a mandrel with many manhours and machinehours.
With the structure that the reinforcement later formed by sheet winding is located at the outermost layer, when a grinding operation is performed, a part of the reinforcement layer is ground, resulting in a reinforcement effect being reduced.
To eliminate the foregoing drawback, a method of producing a golf club shaft by forming a reinforcement layer merely by employing a filament winding process has been discussed. Specifically, this method is practiced such that as shown in FIG. 4, after a fiber layer 2 is formed around the a mandrel 1 by employing a filament winding process, a reinforcement layer 3 is partially formed by the filament winding process prior to curing, moreover, filament winding is performed over the whole length, thereafter, these layers are cured, and then, the mandrel 1 is disconnected.
With such method, since the reinforcement layer can be obtained merely by employing the filament winding process, this method is practicable. However, as shown in FIG. 4, due to a necessity for winding filaments by several turns on the opposite ends of the reinforcement layer 3 under a condition that the winding angle of .theta. as shown in FIG. 3 is set to 90.degree. (in order to prevent the wound filaments from becoming loose), there arises another drawback that a raised portion 31 is formed. In addition, there arises another drawback that a boundary 4 between the reinforcement layer 3 and the fiber layer 2 has a reduced diameter because of the filament winding performed when the reinforcement layer 3 is formed. Thus, a large stepped part is formed between the reinforcement layer 3 and the fiber layer 2. Because of the presence of the large stepped Part, in practice, the golf club shaft can not be sold as a commercial good.